1. Field of the Invention
The invention relates to the production of hydrocarbons from an underground formation. More particularly, the invention relates to testing earth formations to determine formation pressure.
2. State of the Art
The previously incorporated co-owned U.S. Patents describe technology used in the assignee's commercially successful borehole tool, the MDT (a trademark of Schlumberger). The MDT tool is a wireline tool which includes a packer and a probe which enable the sampling of formation fluids and the measuring of pressure transients during sampling or a pretest. One can infer formation permeability from a pressure transient. In addition, the formation pressure can be obtained with the MDT tool by extrapolation from the pressure transient or, preferably, by waiting long enough for the measured pressure transient to stabilize.
Prior art FIG. 1 illustrates an MDT tool as described in previously incorporated U.S. Pat. Nos. 4,936,139 and 4,860,581. The MDT tool 10 is shown in a borehole 12. The tool 10 includes an elongated body 14 that carries a selectively extendible fluid admitting assembly 16 and a selectively extendible tool anchoring member 18. The illustrated tool also has at least one fluid collecting chamber 20 which is coupled to the fluid admitting assembly 16 by a flow line bus 22. The fluid admitting assembly 16 includes a packer 24, a pair of pistons 26 and a front shoe 28 connecting the packer to the pistons. A filter 30 extends through the packer and the front shoe to a filter valve 32. The valve 32 is selectively fluidly coupled to the collecting chamber 20 by the flow line bus 22 which is also connected to a strain gauge 34, a crystal quartz gauge (CQG) 36, a resistivity/temperature cell 38, and a pretest chamber 40 via an isolation valve 42 and an equalizing valve 44.
In order to make accurate analyses of the formation, it is desirable to obtain many pressure measurements throughout different parts of the formation. In addition, because of the expense involved in keeping the MDT tool deployed in a borehole, it is desirable that measurements and samples be taken as quickly as possible. For high permeability formations, the MDT tool provides formation pressure measurements reasonably quickly, two to three minutes per point, much of this time being taken to anchor the tool. For low permeability formations, however, it may take several more minutes for the pressure to stabilize. It will be appreciated that the steps involved in taking pressure measurements include raising or lowering the tool to a desired location, extending the telescoping pistons and the packer to anchor the tool, extending the fluid collecting filter up to the wall of the formation, pumping to remove mud cake and ensure hydraulic communication with the formation, waiting for the pressure to stabilize, then retracting the packer and pistons before moving to the next measurement location.